sisus: SISUS: Stable Isotope Sourcing using Sampling

SISUS for source partitioning using stable isotopes.
SISUS reads in a specific Excel-like workbook and performs an IsoSource-type analysis by
returning a sample of feasible solutions to p in the non-over constrained linear systems, b=Ap.
Edit \file{sisus_*_template.xls} and input data values and run parameters.
Run \code{\link{sisus.run}(filename)}. See output in current working directory.